1. Field of the Invention
The present invention relates to a method of manufacturing a multilayer circuit board and particularly to a method of manufacturing a circuit board of multiple thick film layers to be utilized in a hybrid integrated circuit.
2. Description of the Prior Art
Conventionally, for a multilayer circuit board to be utilized in an LSI package and the like, a high degree of integration, namely, formation of a fine connection pattern is required. A manufacturing method of such a multilayer circuit board is disclosed for example in Japanese Patent Laying Open No. 93296/1983.
In the latest technological trend, large scale integration is required for a hybrid integrated circuit and accordingly a high degree of integration and high reliability are needed in a multilayer circuit board to be utilized in such a hybrid integrated circuit. Such a recent trend concerning hybrid integrated circuits is shown for example in "Polyimide Dielectric on Hybrid Multilayer Circuits (by John Shurboff, Motorola Incorporated, in page 610 of IEEE, 1983)". In order to attain such large scale integration, it is necessary to apply a large number of signal lines in a multilayer circuit board utilized in a hybrid integrated circuit and to provide low resistance signal lines. Since a hybrid integrated circuit including the peripheral devices thereof needs to be functionally operated, it is necessary to obtain a high reliability and a high degree of integration by use of a thick film resistor of high reliability as well as by use of a conductor having good adhesion to a ceramic substrate.
FIG. 1 is a sectional view of a multilayer circuit board manufactured by a conventional manufacturing method. Referring to FIG. 1, a thick film as a first conductor layer 2 is formed on a substrate 1 of alumina ceramic for example, by printing and firing a conductor paste. A thick film resistor 3 is formed on the first conductor layer 2 and an insulating layer 4 of crossglass is formed on the thick film resistor 3. Then, by printing and firing a conductor paste, a thick film as a second conductor layer 5 is formed over the substrate 1, the first conductor layer 2 and the insulating layer 4.
Thus, thick films are formed as the respective layers in a conventional multilayer circuit board to be utilized in a hybrid integrated circuit, whereby the reliability with respect to a thick film resistor can be obtained. However, it is well known that in the thick films thus formed, the limit of a width of a fine pattern is 100 to 200 micrometers. Accordingly, in a well known method, thick film conductors are formed in a subtractive manner by etching or laser firing so that a fine pattern may be formed. However, in order to form a thick film conductor in a subtractive manner, it is necessary to form in advance a thick film conductor in a non-fired or fired state over the whole surface of a substrate. Particularly, in case of a substrate containing a resistor, it is difficult to form a second conductor in regions close to the resistor by a thick film printing, which makes it substantially impossible to form a fine pattern. In the case of a substrate on which resistors are formed, precious metal materials such as silver-palladium, platinum-silver and the like are generally used as a thick film conductor material, which involves a disadvantage that the conductor sheet resistance value is as high as 20 to 100 milliohms in a thick film conductor formed of such a thick film material, exerting unfavorable influence on the signal transmission system.